In home security, magnetic circuit breakers are often used to detect that a window/door is closed. In doing so, in essence the magnetic field along 1 direction (typically perpendicular to the IC surface) is compared with a user-defined threshold. This system provides good functionality, as contact-less magnetic position sensing is robust against dirt, which could otherwise affect other technologies such as optical circuit breakers, and it is also robust against wear & tear, as contacting solutions such as microswitches or mechanical relays. FIG. 1 illustrates a conventional setup for a circuit breaker.
As such, magnetic circuit breakers are very popular in home security systems and other (non-) secure closure detection systems, such as white goods closure detection and emergency exits.
Typically, magnetic circuit breakers are characterized by the fact that the switching threshold is either factory trimmed at the sensor's IC manufacturer, or it is trimmed end-of-line at the module or equipment manufacturer. This means that the switching accuracy is still exposed to any dynamic variations over time: window not closing as well as it used to or air gap change over temperature and time.
The principle of operation is as follows. In essence the switching point Bop is crossed when the window is closed and then the alarm will go off (OFF state) whenever the magnetic field has dropped below the BRP point. Bop and Brp can be the same value, but typically hysteresis is intentionally introduced so as to avoid false tripping back and causing chattering condition (switching in and out all the time due to noise). The latter is illustrated in FIG. 2. The comparison function of a switch (reed sensor or CMOS magnetic sensor) in systems used is typically a 1-bit information to check if the magnetic field is higher than a predefined threshold in absolute value. This means that the accepted region (switch considered closed) is open-ended, and not a narrow region. In essence the magnetic circuit breaker will compare an absolute threshold (be it a factory trimmed one or programmed end of line) with the measured magnetic field. Malicious people could take advantage of that by applying an additional external magnetic field, making the sensor believe the magnet is still in front of him when the magnet is pulled away. The external tampering field just needs to be at least as big as the one induced by the magnet from the circuit breaker and then the system is fooled that the window is still closed.